My thoughts on this image and I would be interested in other opinions.

There is no primary return in white so I don't believe this is the shadow of the crocodile. I think maybe the sound wave is absorbed by the body of the crocodile and so there is no return and the image is not a shadow but an absence of a reflection. When crocodiles swim in the midwater they swim with their tail and their legs are tucked in. In this image the croc has its legs out as if it is crawling along the bottom. This being the case we might expect some white object close to the black image but their is none. I guess I am suggesting that an object can absob the sound pulse and so may look like a shadow but is in fact the primary object itself.

John,
I've wondered this myself as I've tried to image an american Alligator several times but failed to see anything on the display that I could make out as a 'Gator. Mine were all with the 'Gator against a busier background so maybe a dark image such as you have was there but I could not detect it? I've been around many 'Gators (Father-in-law trapped them for Geogia DNR) and now wonder if their hide absorbed more than reflected sonar. I know it sure feels hard enough that you would think it would give a really good reflection! I've seen the same things when looking at sonar images of other animals. As far as the spread legs goes: we've seen them do this when they are motionless in the water column too and not just on the bottom.

True, when our freshwater crocodiles are motionless they do indeed spread their legs although usually when we are around the saltwater crocodiles they are very rarely motionless when we are close to them.

I did have another two images which unfortunately I did not take photos of. Both showed the triangular tips of the leather of the tail and along the body in white so a good return. Obviously these bits of leather were hard enough to give a good return. When I zoomed in I could see the faint blackness of the crocodle underneath but clearly visable as a crocodile. It initially looked like a spine of a creature till I looked more closely and saw what it really was.

I am going back to this river in 6 weeks and now have my machine writing to the SD card so will have some good shots to show.

Thank you for the images John, they have created quite a buzz here. Please do post any screen captures when you make them in the future, we would very much like to see more.

I do believe that you are seeing the shadow of the croc, and I have a few educated guesses as to why you are not seeing the primary return.

1. Crocs are very faceted, and it is certainly possible that much of the impinging sound wave is dispersed by the textured skin of the gator. The features on a croc/gator are acoustically large compared to a 455kHz wavelength and would be excellent at scattering the sound wave, much like a stealth jet scatters radar pulses. I would not expect this to cause the croc to totally disappear, as there would still be a large percentage of directly reflected sound. This leads to point no. 2.

2. We implement time variable gain, where the signal is amplified more greatly as time increases from the transmit pulse. It is certainly possible that the croc is very close to your boat and the dispersion described in (1.) above is combining with the low gain at close distances to effectively erase the primary return. This would be an unusual circumstance; most objects even very close to the boat are imaged well, as those who observe fish in the water column will report.

An alternative explanation is that like you suggested, the croc is very close to the bottom and instead of directly reflecting the sonar pulse, his faceted features are scattering the sound, causing a returnless shadow at his location. In that case you would be correct that the shadowed area is in fact the primary target. The only reason that I don't fully subscribe to this explanation is that there is such a contrast between the bottom and the shadow, which I have never seen on a directly reflected return. If you look at most direct returns of textured objects like boulders there is less contrast among the features than your image shows.

Whatever the reason, it is a very curious occurrence and I would like to see any more pictures that you can come up with, especially showing a non-zoomed window.

Thank you again for taking the time to post these intriguing pictures.

Of more value might be a short Sonar Recording if you can do so John. I'm sure David and the other guys here would be real interested in that - I know I would be! Not sure how you would talk the Big Lizards into cooperating though...

Here is a non-zoomed image of the first one I posted it is a very focussed digitital shot, some much so you can see the individual pixels.

All the shots I have posted were in pretty shallow water, 10 foot and less. This one looks like it was 2.1m.

Yes in discussiong things further over here I considered the stealth croc and I am sure there is some validity in that. I am more inclined to the primary return idea. As I mentioned in a previous post I saw two other screens where the tips of the crocs along their back where shown in white and the black of the croc underneath. You could clearly see the white tips and the shadow were part of the same image. But the dark part was not as dark or contrasted as as these images so maybe the jury is still out. Thanks for your interest in my shots. I now have the 8gig card working so do intend to take lots of shots and recordings on my next trip. I think this technology is fasinating. To me colour sounders have gone about as far as they can and side sonar can only develop. I suppose the crocs just help us understand better how things work.

The higher frequency 800 kHz sonar signal appears to be completely attenuated(scatted and absorbed by the target)the sonar signal,and there is no returning sonar echo data to be processed by the your unit.It only showing a complete loss of the signal,none being returned at all!

It would be very interesting to see a image done at 262 kHz with a 1st generation SI transducer.I feel it would return a echo of the target,at that lower frequency. And therefore also yield a shadow image on the bottom, that would allow you to determine a height off the bottom of the target.This i have found is to be very useful in my use of SI.

Also rather than post a picture of your unit's screen,could you post a captured screen shot by your unit. It would allow a more detailed image to be examined,by all of use here.

Very interesting results,you have obtained,please add to you results, as you obtain more data!!!

Last edited by tuna1 on Wed May 21, 2008 12:12 pm, edited 1 time in total.

My new 997 wouldn't read the card so I had to resort to the camera. I have since replaced the unit and can now take screen shots and recordings. I will be back on the river in 5 weeks time for 3 whole weeks so hope to get more shots of our salties to post here when I get back. I will also do more experimenting with the 455 freguency. The crocs shots were all in 7 to 10 feet of water.

This stuff is certainly great technology which I am enjoying learning.

Great work,very interesting!!! Please keep us all informed about your progress. Leave it to Mother Nature to surprise use,a target that presents no returning sonar echos-completely absent of any returns... That's a good trick!!!!

Regarding the 455kHz beam, so what you are saying is this frequency is a "stronger" beam which will give more chance of resulting in a return from an object like a a faceted and/or sound absorbing croc? You may have caught my other post regarding the spine looking return on other crocs. Unfortunately I did not take a photo. They looked like the long spine of the total length of the crocodile and when I zoomed in you could see the darker shape of the croc below. The proximity and position of the white, spine like return ,to the darker immage lead me to conclude I was seeing the primary target. The hard tips of the tail and back of the croc in white and the dark absence of return of the either sound absorbing or reflecting croc. I am ging to try to capture that image on my next trip in 5 weeks.

The 800kHz beam, while having a very short wavelength and allowing for great resolution, will also be attenuated more easily by turbid water and biological mass. Looking at your photos of the unzoomed image (I got your email), I am leaning more towards the theory that the croc has scattered or absorbed most of the 800kHz pulse. As you accurately observed, there does not seem to be any sign of a typical primary return.

The 455kHz beam will penetrate further into the water, and will be less likely to scatter than the 800kHz beam. Perhaps you could capture some images of each beam when you make your next excursion?